The invention relates to a method for increasing the coolant side surface roughness on castings by electrical discharge machining (EDM). More particularly the invention is directed to a method for increasing the coolant side surface roughness on interior cavity surfaces of airfoil castings by using EDM.
As is well known to those skilled in the art gas turbine airfoils operate in an extremely hot environment. In order to meet the life requirements of the machines, compressor discharge air is circulated through the airfoils to cool them. However, bleeding air from the compressor lowers the overall turbine efficiency. Therefore, engineers have been challenged by the life requirements of the airfoil and the need to minimize the use of cooling air.
Recent developments in heat transfer technology have shown that through wall heat fluxes can be improved by increasing the surface roughness of the coolant side wall, thereby reducing the amount of cooling air required to meet the airfoil life goal. The present invention increases internal surface roughness of turbine airfoil castings through the use of EDM.
An electrode in the shape of the entire internal core cavity or proportions thereof can be utilized to effect the internal surface roughness. The specifically shaped electrode is then used to machine the inside of the core cavity creating a roughened surface.